1. Field of the Invention
The present invention relates to an airtight terminal and a method for producing the same, a piezoelectric vibrator having an airtight terminal and a method for producing the same and an oscillator, an electronic unit and a wave timepiece having a piezoelectric vibrator.
2. Description of the Related Art
Piezoelectric vibrators are indispensable to the production of industrial products such as timepieces, oscillators, electronic units and wave timepieces. Piezoelectric vibrators are used timekeeping sources, timing sources or references for signals. Piezoelectric vibrator packages commonly used include box-type ceramic packages and cylindrical cylinder-type packages. The configuration of the latter or a cylinder-type package will be described briefly with reference to FIGS. 23A and 23B.
FIGS. 23A and 23B are pattern diagrams showing the configuration of a conventional cylinder-type piezoelectric vibrator. For example, a vibrating piece 8 such as a quartz vibrator is secured to the inner lead 3 of a lead 2 for the airtight terminal 1 by means of plating using a metal film and a conductive adhesive, which are not shown. The vibrating piece 8 is also covered by a bottomed cylindrical metallic case 10 and airtight sealed to provide a vacuum. The outside perimeter of the airtight terminal 1 is tight fit to the inside perimeter of the case 10. In FIGS. 23A and 23B, it is assumed, for the purpose of describing the construction of the inside thereof, that the case 10 is a transparent body. Note that the side of the lead 2 to which a device thereof is connected to is herein referred to as an inner lead 3 and the side thereof which is mounted on a substrate as an outer lead 4.
The airtight terminal 1 is filled with a filler 6, which is used for hermetic sealing in an outer ring called a stem 7. Two parallel leads 2 each composed of a thin solid metal round bar are inserted through and fixed to the filler. The surfaces of the lead 2 and the stem 7 are plated. The inner lead 3 is connected to the vibrating piece 8 through plating by melting a local plating of the surface of the inner lead 3 and securing the lead 3 to a mount pad 9 formed at the base of the vibrating piece 8. The mount pad 9 serves as a connection region between the inner lead and the vibrating piece 8. The case 10 is also mounted to the stem 7 from above the vibrating piece 8 along the contour thereof. The case 10 is airtight bonded to the stem 7 by means of cold pressure welding through a plated layer 12, a soft metal which the outer ring of the stem 7 is made of. Note that in FIGS. 23A and 23B, the thickness of the plated layer 12 is exaggerated. Conventionally, the process of manufacturing piezoelectric vibrators using such packages is automated.
However, rapid growing reduction in the size of parts in recent years has made it very difficult to produce piezoelectric vibrators using conventional piezoelectric vibrator production methods with good yield and at low costs. One of the main problems related to the difficulty is a rise in unit price of the airtight terminal due to a drop in plating yield in the airtight terminal production process. The second main problem is a drop in rigidity of the outer lead of the airtight terminal. The third main problem is a fluctuation in resonance frequency and resonant resistance values after airtight sealing. These three problems will be briefly described below.
The first problem, a substantial drop in plating yield arising from reduction in airtight terminal size, is caused by a drop in rigidity due to a reduction in the interval between leads mentioned above and a decrease in a lead diameter. The diameter D of a cylinder-type vibrator is reduced from approximately 3 mm to 2 mm or further to 1.5 mm. The diameter D serves as a maximum value for the contour of the case after sealing. The diameter for cylinder-type vibrators used in recent cell phones is smaller, that is, 1.2 mm. With the growing tendency of smaller vibrator diameters, the employment of such vibrators with a diameter of less than 1 mm is also under consideration. Because of the growing tendency of smaller such vibrators, the interval d1 between two leads, component members of the airtight terminal, is extremely small and the diameter d2 of the lead itself is smaller, thus resulting in a drop in rigidity. Therefore, the lead may more easily bend.
A conventional plating process employs a barrel plating method advantageous for mass production. The shape of a barrel is a hexagonal column having a diameter of a few dozen cm and a length of approximately 40 to 80 cm, for example. The material of the barrel is a resin such as acrylate. Approximately 200 thousand to 500 thousand airtight terminals are placed in the barrel, which is then placed in a plating bath. Airtight terminals are then electroplated over a time period of a few hours while the barrel is being rotated slowly in the bath to agitate the airtight terminal therein. In the process, small airtight terminals with a diameter of 1.2 mm or less frequently suffer from failures such as two leads connected together through plating and contact of one outer lead with another because of the small interval between leads is small and the fact that leads themselves can easily bend. This causes problems such as a substantial drop in plating yield and a great rise in airtight terminal production costs.
The second problem is a drop in rigidity of the outer lead of the airtight terminal. A drop in rigidity of the outer lead has also been pointed out as a problematic point about the first problem, which is related to the production of airtight terminal. The reduction in rigidity is herein considered as a problematic point having another aspect occurring in the piezoelectric vibrator assembly process. This problem is associated with the inner lead, which comes from a reduction in size of the vibrating piece.
A further reduction in size of future vibrating pieces will lead to a reduction in both the area of the mount pad on the vibrating piece and the clearance between the vibrating piece and the inner lead. Consequently a possible challenge will be how to establish an accurate positional relationship between the tip of the inner lead of the airtight terminal, which is bonded to the mount pad, and the mount pad. In addition, outer leads are mechanically arranged and held on a pallet in the assembly process, as will be described later. Outer leads therefore need to be sufficiently rigid to withstand a bend during the assembly process. For conventional airtight terminals, the entire lead is formed of a solid round bar with a uniform diameter because leads formed of such a round bar are easy to produce. Conventional methods have so far used the inner lead with a smaller diameter in response to a smaller mount pad area in the vibrating piece, thus resulting in improved accuracy in alignment between the mount pad and the tip of the inner lead. Assuming, however, that conventional methods are followed, the diameter of the outer lead will also be smaller, thus causing the outer lead to be insufficiently rigid. For example, the use of an inner lead with a diameter of 50 μm for a mount pad with a width of 50 μm will cause the lead to easily bend. Obviously this will make the lead insufficiently rigid. There is a concern that airtight terminals of a conventional construction, which have the same diameter for the inner and outer leads, could not sufficiently withstand a reduction in the size of vibrating pieces.
The third problem is a fluctuation in resonance frequency and resonant resistance values for vibrators after airtight sealing, which fluctuation is caused by those sealing requirements in the vibrator production process restricted by gases leaving jigs used in the process. The fluctuation is greatly affected by the use of particularly pallets made of resin. Pallets are holding jigs for arranging a plurality of airtight terminals thereon and causing the airtight terminals to flow from the first to last steps in the vibrator assembly process. Pallets need meet both the mechanical requirement of holding airtight terminals securely and with good accuracy and the electrical requirement of serving as an insulator that prevents electrical interference between adjacent airtight terminals. Because an extremely large number of pallets are also required in the mass production process, pallets formed of resin have been used considering their ease of procurement and disposal including their costs.
FIG. 24 is a schematic pattern showing a conventional pallet and the arrangement of airtight terminals thereon. Outer leads 4 of the airtight terminals are arranged at constant intervals while mechanically held by means of metal terminals 36 attached to the pallet 35. There is also electrical conduction continuity provided between the metal terminal 36 and the lead. Being made of resin, pallets can be easily constructed to be capable or receive metal terminals 36. For vibrators—for example, tuning fork type quartz crystal vibrators—which are sealed in a vacuum atmosphere, however, the seal atmosphere is heated to a high temperature, at a capping step (other called like a press-fitting step or a sealing step), to remove moisture absorbed during assembly from the vibrators and to release as many gas components from each member. This also makes pallets hot and gases are generated by resin, thus resulting in a drop in airtight-sealing vacuum, which causes a fluctuation in resonance frequency and resonant resistance values over time. In addition, the small calorific capacity of a small vibrator makes the vibrator easy to become hot in a reflow process performed by a customer. If vibrators are mounted on a substrate particularly through lead-free solder, the vibrators are at a temperature above 260° C. at a reflow, thus causing the vibrators to suffer from a large change in resonance frequency and resonant resistance values. Consequently, in the production process, the capping step need be performed at high temperatures to prevent a fluctuation in frequency and resonant resistance values at the above-mentioned reflow. However, conventional pallets have a problem that increasing the temperature increases the amount of gases released, which necessitates the consideration of pallet materials.